The disclosed invention relates in general to fiber coatings and more particularly to a coating method and furnace suitable for applying on-line at high pull speeds a corrosion resistant hermetic coating on optical fibers. It is well known that bare uncoated optical fibers are susceptible to corrosion by various chemicals, including water. Cracks and scratches in the fiber surface present regions susceptible to chemical attack, especially when the fiber is under stress. Therefore, optical fibers are typically coated with an abrasion resistant coating to prevent scratching the fiber. However, the optical fiber surface typically contains microcracks, produced during manufacture of the fiber, that can also be attacked by water. Therefore, to prevent sudden breakage of a fiber due to attack by water, it is important in many applications to apply a hermetic coating to the fiber. The hermetic coating as well as an abrasion resistant coating must be applied on-line as the fiber is being pulled so that the fiber is protected before it is wound onto a take-up spool. Therefore, a process is required that can deposit hermetic and abrasion resistant coatings at fiber pulling speeds appropriate for producing production volumes of optical fiber--namely, at speeds on the order of 1-10 meters per second.
In borehole logging operations using optical fiber, the optical fiber can be exposed to water at up to 200 degrees Centigrade and 20,000 psi. In addition, the weight of the instruments attached to the optical fiber and the weight of the metal cables used to support the instruments results in up to a 3% strain in the metal cables and in the optical fibers. Under such temperature, pressure and strain, it has been found that an uncoated fiber breaks within seconds. Therefore, a hermetic coating is needed that can protect an optical fiber under these hostile conditions.
In U.S. Pat. No. 4,512,629 entitled OPTICAL FIBER WITH HERMETIC SEAL AND METHOD FOR MAKING SAME, issued to Eric G. Hanson, et al on Apr. 23, 1985, a coating containing silicon and carbon is presented that has proven to be a hermetic coating under the hostile conditions experienced by an optical fiber in borehole logging. As indicated in that application, the addition of silane (SiH.sub.4) to the carbon source reactants increases the reaction rate resulting in a thicker coating than without the addition of silane. However, a chemical resistance test in which a fiber is immersed in hydrofluoric acid indicates that as the amount of silicon in the coating is reduced, the chemical resistance increases. Also, fast fracture tests indicate that there is a small increase in fiber strength as the amount of silicon in the coating is decreased. However, the reduction of the fraction of silane in the reactants reduces the thickness of the resultant coating. As indicated in U.S. Pat. No. 4,512,629, it was found that the elimination of SiH.sub.4 resulted in a coating that was not hermetic. It has been speculated that the coating was not hermetic because it was too thin. Therefore, a method and furnace are needed that can produce extremely high deposition rates so that the amount of silicon can be reduced without unduly reducing the thickness of the resultant coating. Such a method should enable the on-line deposition of pinhole-free, strongly bonded coatings on optical fibers at commercial pulling speeds. The order of magnitude increase in the pulling speed for a production process requires that a process be found that has more than an order of magnitude increase in the rate of deposition of carbon on an optical fiber compared to the method disclosed in U.S. Pat. No. 4,512,629.
In FIG. 1 is shown a CVD furnace suitable for use in coating optical fibers. That chamber is disclosed in Japanese patent application No. 54-151947 which was laid open on June 7, 1980 in publication 55-75945. That CVD furnace has a reaction chamber 11 that has a reactant inlet 12 and an exhaust port 13. That furnace also has a fiber inlet 14 and a fiber outlet 15 for passage on-line of an optical fiber 10 through the reaction chamber. In order to prevent reactant gases from escaping through the fiber inlet or the fiber outlet, gas seals 16 and 17 are located at the fiber inlet and at the fiber outlet. Seals 16 and 17 respectively have inlets 111 and 112 through which an inert gas is supplied and have apertures 18 and 19 through which the fiber is pulled. Gas seals are utilized as the seals at the fiber inlet and the fiber outlet to avoid scratching and/or contaminating an optical fiber due to passage through the seals. Although the particular embodiment shown in FIG. 1 utilizes heating coils 110 to heat the walls of reaction chamber 11 for a hot wall CVD process, the specification also indicates that other methods of heating can be used including rf heating, laser heating of the fiber or placing the reaction chamber near enough to the meltdown point at which the fiber is pulled from the preform that the fiber is hot enough for a cold wall, hot fiber CVD coating process to take place.
Unfortunately, the use of gas seals at both ends of the reaction chamber draws into the reaction chamber ambient gases entrained by the moving fiber. The problems arise primarily at aperture 18 in gas seal 16. When a fiber is pulled through the reaction chamber, a layer of ambient gases at the surface of the fiber is drawn through gas seal 16 into the reaction chamber. At low speeds with a sufficiently long gas seal 16, this layer of air has time to diffuse away from the fiber before the fiber enters the reaction chamber so that only a negligible amount of ambient gases is drawn into the reaction chamber. However, in many applications, the top seal 16 is not long enough to remove the entrained gases before the fiber traverses the entire gas seal. Therefore, a method of deposition is needed which prevents an unacceptable amount of ambient gases from being drawn into the